Hi - I don't believe there is so yet. I know Vazgen and Resource Mgmt Product team have talked about putting the planned hours data by day (including allocations changes made by a resource manager) in the API, but not until they finish all their work on the Workload Balancer. So I don't think it is something you will get anytime soon.
Now, with that said... if you are looking for just planned hours by client (I'm assuming that is portfolio or some Project attribute), and he have at least one Legacy Resource Pool still (not sure how new you are), you can use the Resource Estimate object type for by Week and by Month (The resource estimate object type splits hours up by weekly allocation date. It is from the old flash-based Resource Estimate section on the business case).